RFR: 2328: Ignore bot user's comment when determining the latest action time of a PR

[Zhao Song]

PullRequestPrunerBot is responsible for closing PRs that have not been active for a certain time. However, it currently doesn't ignore comments from bot users when determining the latest action time of a PR.

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Commit messages:
 - SKARA-2328

Changes: https://git.openjdk.org/skara/pull/1687/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1687&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2328
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1687.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1687/head:pull/1687

PR: https://git.openjdk.org/skara/pull/1687